No. I.] GAR-PIKE AND STURGEON. 41 



bridging its hinder margins forms an imperfect roof for the 

 neurenteric canal. The elliptical margin of the blastopore 

 appears thick and opaque (PI. IV, Fig. 66). From it, passing 

 forward and slightly diverging, terminating near the hinder 

 region of the head, are the pronephric ducts. The closure of 

 the blastopore and the detailed establishment of the neurenteric 

 canal may at this point be most conveniently understood. A 

 condition later than that of PL IV, Fig. 64, is given in the same 

 plate, Fig. 65 : it illustrates the continued reduction in diameter 

 of the yolk plug and its great increase in (ecto-entad) length : 

 the inflected rim of the blastopore is now exceedingly deep, the 

 recessus (Kupffer's vesicle) under the dorsal lip, prominent, 

 deep but somewhat rounded : a general growth in extent of the 

 entire outer wall of the coelenteron appears to have taken place 

 since its distance from the yolk is seen to have greatly in- 

 creased. At a subsequent stage. Fig. 66, when the blastopore 

 has become reduced to but half the diameter of that last 

 figured, the yolk plug loses its distinct cylindrical character : 

 it comes to conform to every irregularity of the thickened 

 blastopore. Further reduction of the diameter of the yolk 

 plug takes place regularly : in Fig. 6^, it has become greatly 

 narrowed, and as the section is slightly oblique its con- 

 nection with the neural canal may be seen : it still touches 

 the surface at a narrowed point. An outward view of these 

 conditions is to be seen in PI. Ill, Fig. 53 : the deepest point 

 of the indentation between the tail folds is the entrance of the 

 neurenteric canal ; immediately below it, at the darkly shaded 

 point, is the disappearing remnant of the yolk plug. In this 

 figure may in addition be seen the hinder limits of the parietal 

 zone, the outline of the neural axis, the undifferentiated tissue 

 of the primitive segments, and at the embryo's extreme margin 

 the hinder part of the pronephric ducts : the tail folds are 

 greatly flattened, but at the hindermost margin are slightly 

 raised above the surface. A final stage in the fate of the 

 blastopore is to be seen in Fig. 54 : the tail folds have 

 fused in the median line, leaving a slit-like opening into the 

 neurenteric canal to be seen at the surface. In sections, PL 

 IV, Fig. 68, and Fig. 69, it seems evident that the lower part, 



